1. Field of the Invention
The present invention relates to a method for making a lithographic printing plate whereby a lithographic printing plate precursor is image-wise exposed and developed in a first and second developing solution, provided in a cascade system.
2. Description of the Related Art
In lithographic printing, a so-called printing master such as a printing plate is mounted on a cylinder of the printing press. The master carries a lithographic image on its surface and a printed copy is obtained by applying ink to said image and then transferring the ink from the master onto a receiver material, which is typically paper. In conventional, so-called “wet” lithographic printing, ink as well as an aqueous fountain solution (also called dampening liquid) are supplied to the lithographic image which consists of oleophilic (or hydrophobic, i.e. ink-accepting, water-repelling) areas as well as hydrophilic (or oleophobic, i.e. water-accepting, ink-repelling) areas. In so-called “driographic” printing, the lithographic image consists of ink-accepting and ink-adhesive (ink-repelling) areas and during driographic printing, only ink is supplied to the master.
Printing masters are generally obtained by the so-called computer-to-film (CtF) method, wherein various pre-press steps such as typeface selection, scanning, color separation, screening, trapping, layout and imposition are accomplished digitally and each color selection is transferred to graphic arts film using an image-setter. After processing, the film can be used as a mask for the exposure of an imaging material called plate precursor and after plate processing, a printing plate is obtained which can be used as a master. Since about 1995, the so-called ‘computer-to-plate’ (CtP) method has gained a lot of interest. This method, also called ‘direct-to-plate’, bypasses the creation of film because the digital document is transferred directly to a printing plate precursor by means of a so-called plate-setter. A printing plate precursor for CtP is often called a digital plate.
Digital plates can roughly be divided in three categories: (i) silver plates, which work according to the silver salt diffusion transfer mechanism; (ii) photopolymer plates which contain a photopolymerizable composition that hardens upon exposure to light and (iii) thermal plates of which the imaging mechanism is triggered by heat or by light-to-heat conversion. Thermal plates are mainly sensitized for infrared lasers emitting at 830 nm or 1064 nm. Photopolymers can be sensitized for blue, green or red light (i.e. wavelength range between 450 and 750 nm), for violet light (i.e. wavelength range between 350 and 450 nm) or for infrared light (i.e. wavelength range between 750 and 1500 nm). Laser sources have been increasingly used to expose a printing plate precursor which is sensitized to a corresponding laser wavelength. Typically, an Ar laser (488 nm) or a FD-YAG laser (532 nm) can be used for exposing a visible light sensitized photopolymer plate. The wide-scale availability of low cost blue or violet laser diodes, originally developed for data storage by means of DVD, has enabled the production of platesetters operating at shorter wavelength. More specifically, semiconductor lasers emitting from 350 to 450 nm have been realized using an InGaN material. An infrared laser diode emitting around 830 nm or a Nd-YAG laser emitting around 1060 nm can also be used.
After image-wise exposure the printing plate precursors are processed with a developing solution to remove the coating from the support at non-printing area, revealing the underlying hydrophilic surface of the substrate. These non-printing areas are the non-exposed areas for negative-working printing plate precursors and the exposed areas for positive-working printing plate precursors. The coating in the non-exposed areas for negative-working printing plate precursors is not hardened and the coating in the exposed areas for positive-working printing plate precursors has an increased solubility in the developer than in the non-exposed areas. The most common way to remove the coating in the non-printing areas is to contact the image-wise exposed precursor with a developing solution. The developing solution, hereinafter also referred to as developer, can be an aqueous solution or a solvent based solution. Typically, the developer is an aqueous alkaline solution or a gum solution which can be used for developing and gumming the plate simultaneously in one single step.
During processing the developer becomes loaded with components of the coating that have been removed during development and the amount of material in the developer increases as more precursors are developed. Due to the increasing amount of material in the developer the activity of the developer decreases resulting in a changed ability to remove the non-printing areas of the image-wise exposed precursors. This means that, as more precursors are developed, precursors are not developed in the same way, resulting in changed lithographic properties of the printing plates.
It is important for a high-quality printing platemaking system that the lithographic properties are as much as possible at a constant level for each processed plate. This means that also when starting a developing process with fresh developer, the activity of the developer has a constant activity level or reaches a constant activity level after developing a limited number of plates.